1. Field of the Invention
This invention relates generally to an electronic tape measure incorporating a digital readout and, more particularly, to an electronic digital display tape measure incorporating an optical encoder for providing a measurement of distance having an accuracy of about 0.01 mm.
2. Discussion of the Related Art
Compact and lightweight tape measures having a metal measuring tape which is secured and wound around a reel within a housing in a spring loaded fashion are known in the art. The conventional tape measure of this type enables a worker to manually extract the tape from the housing against the bias of the spring to a length such that a measurement can be made on a particular workpiece or the like by visually aligning measuring indicia on the tape with a desirable location on the workpiece. Once a measurement has been taken, the tape can be automatically wound back on the reel within the housing by the spring. Clearly, these conventional tape measures are limited in accuracy in the ability to precisely locate the marking indicia, as well as the ability of the worker in visually aligning the indicia with the desirable location on the workpiece.
In order to, at least in one respect, provide heightened accuracy over the conventional tape measure, it is also known in the art to include certain mechanisms associated with the tape measure for automatically determining the length of the tape as it is extracted from the housing, and also to provide a readable indication of this length. A worker need only line up the length of the tape extracted with the distance to be measured to make a measurement.
A number of mechanisms are known in the art for determining the length of the tape as it is being extracted from the housing. In many cases, these mechanisms include circuitry for producing pulses which are converted into a linear measurement. Different pulse producing circuits include providing conductive brushes in contact with a conductively encoded disk rotatable as the tape moves, providing optical sensors, in one form or another, for measuring rotation of a disk as the disk is rotated when the tape moves, and providing a magnetic strip along the tape which is read by a magnetic strip reader as the tape is extracted or retracted into the housing.
In one particular application, U.S. Pat. No. 4,551,847 issued to Caldwell discloses a digital tape measuring device in which a measuring tape includes a series of spaced holes. As the tape is wound or unwound from a spring loaded reel, the holes engage a rotatable pinned sprocket. As the sprocket rotates with the movement of the tape, an encoded disk rotates with the sprocket. Optical sensors are mounted relative to the encoded disk which are optically responsive to indicia strategically positioned on the disk. As the disk rotates, the optical sensors provide a linear displacement of the tape with an accuracy of 1/32 of an inch.
Although the prior art digital tape measuring devices offer improvements over the conventional manual tape measuring devices, the accuracy with which these tape measures can measure a distance can be improved. It is therefore an object of the present invention to provide a digital tape measuring device which includes components for providing a higher degree of measuring accuracy heretofore not found in the prior art.